Pokémon

If Platinum was the last game you've played, and you are willing to pick up a Switch, Legends Arceus is a great choice. It's set in the past of the Sinnoh region, features a somewhat decent amount of newer Pokémon, has a good story overall, and is generally agreed upon to be the best Pokémon game of generation 8. The semi-open world and ability to catch Pokemon without battling them (featuring stealth mechanics) make for unique and fun gameplay for the series. You can even complete the pokedex all on one cart without needing to trade.

If you want to pick up a "mainline" non-spinoff game, no better place to jump in than the current generation, in this case Scarlet/Violet. It's the game with the most current stuff going on and with the most active community. Most of the complaints about it are about the graphics and framerate/performance on the Switch hardware, but many also say that despite the performance issues these are the best mainline games in the series from story and gameplay standpoints. There's a lot of online bickering among fans who are both positive and negative on the gen 9 games, but I would say watch some YouTube or Twitch videos of gameplay preferably without commentary to see what the games are like and make up your own mind on it before paying for a Switch and the game.